1. Field of the Invention
This invention relates to digital imaging systems in which moving photographic film is exposed to a focused light beam. More particularly, it relates to such a system having a control mechanism to insure uniform film exposure independent of the rate of movement of the film relative to the focused light beam.
2. Description of the Related Art
In many digital imaging systems, the film is wrapped around a rotary drum. The light beam is either a conventional light beam or a laser beam The light beam is intermittent with an intensity that is modulated in accordance with the information to be represented by the image. The image is formed of small rectangular spots, called "pixels". During the exposure of each pixel, the beam is turned on and off with a predetermined ratio of on-time to off-time Such systems have the disadvantage of being susceptible to any variations in the rotational speed of the drum. Variation in the speed of the drum, changes the ratio of the on-time to the off-time relative to the movement of the film during that period and detracts from the quality of the overall image. Problems also result from variations in the "firing" of the laser beam relative to the speed of the drum resulting in improperly spaced pixels. The usual method employed to treat this problem (either with or without a phased-locked loop) is to utilize each pulse from a rotary encoder operated by the drum to initiate an open-loop timing signal that "fires" the laser beam to expose the film on a pixel by pixel basis. Such a system insures the proper number of exposures for each drum revolution irrespective of changes in the drum speed, but variations in the exposure ratio relative to the drum speed may cause overlapping or separated pixels.